As described in this assignee's U.S. Pat. Nos. 6,837,235 ('235) and 6,915,522 ('522), when a glass-coated steel wire oven rack is subjected to temperatures above 900° F., there is an emission of hydrogen gas from the steel upon cooling from that temperature, and absent a preventive expedient, the emitted hydrogen gas will attempt to escape from the steel through the glass coating causing the glass coating to chip, spall or crack.
There is no solution to preventing the chipping, spalling or cracking of glass-coated steel wire oven racks or of glass-coated drawn steel rod articles, with the exception of the solution described in this assignee's '235 and '522 patents and pending application Ser. No. 11/040,641, filed Jan. 21, 2005.
As described in this assignee's '235 and '522 patents, the drawn steel rod is subjected to at least 20% reduction in diameter during cold drawing; and the rod, at the time it undergoes drawing, is composed of steel comprising up to about 0.08% carbon and about 0.001 to about 0.2% of a carbon stabilizing transition metal selected from vanadium (V), titanium (Ti), niobium (Nb) and tantalum (Ta). This, combination of features enables the glass-coated drawn steel rod article or wire oven rack to overcome the glass chipping or cracking problem as a result of hydrogen out-gassing.
In addition to the hydrogen out-gassing problem experienced at high temperatures with porcelain-encapsulated steel oven racks, another very significant problem has more recently been discovered during the manufacture, testing and use of the porcelain-coated oven racks. It has been found that the porcelain can deteriorate by marring, flaking or chipping off of the porcelain material from the oven racks as a result of the normal periodic sliding contact between the oven rack porcelain surface and a contacting porcelain wall surface of the oven cavity. That is, over the 13 to 15 year normal life expectancy of an oven, the repeated sliding porcelain-to-porcelain contact upon insertion and removal of the porcelain-coated oven racks, particularly when the oven racks are supporting a relatively heavy cooking load, can cause unwanted abrasion, chipping and squeaking of the sliding porcelain surface (of one type) against and across a porcelain surface (of the same or another type) on the oven wall. The identification of a suitable porcelain composition that solves this problem was found to be a daunting task since the porcelain composition must be strong enough to solve the chipping, spalling and fish-scaling problems that may result from the hydrogen out-gassing of the carbon steel as well as resist damage resulting from continued heating and cooling cycles experienced in cooking, and especially the high temperatures of self-cleaning oven cycles, while maintaining sufficient lubricity and hardness to pass enumerable quality tests typically required for a porcelain material to be suitable as an oven rack. For example, a suitable porcelain material for an oven rack must pass a lubrication test; gloss test; adherence test; thickness test; fish-scale test; must be resistant to acids; resistant to alkaline materials; be resistant to crazing; be resistant to abrasion; pass a rubbing test; blurring test; toxicity test; humidity test; specific gravity and corrosion test as well as others. Porcelain quality tests generally are specified in the Manual of Tests, Measurements and Process Controls PEI-1101, an enameling manual well known in the art, hereby incorporated by reference. Even other such tests for porcelain quality are set by ASTM standards.
After-coating the oven rack with a liquid lubricant, such as the prior art method of using vegetable oil, requires repeated reapplication of vegetable oil since the oil dissipates, e.g., burns off, in both continuous-cleaning and self-cleaning oven cycles and also somewhat during other oven usage such as normal cooking cycles. Prior to this assignee's out-gassing solution, as described in the '235 and '522 patents, commercially satisfactory porcelain-coated oven racks to be used in self-cleaning pyrolytic ovens were non-existent so that assistance in attempting to solve the porcelain-to-porcelain abrasion and flaking problem in porcelain materials that are regularly subjected to temperatures above 900° F. was not forthcoming from the prior art.